1. Field of the Invention
The present invention relates to a catamaran.
2. Description of the Related Arts
Catamarans, ships constructed so that two single hulls arranged transversely and spacedly are connected by a deck disposed above the sea level, have been publicly known. A semisubmerged catamaran is one type of catamaran. The inventors have made efforts to improve the semisubmerged catamarans, and as a result have developed a semisubmerged catamaran whose hulls oscillate less in waves and which can navigate on the waves with low required horsepower. This catamaran has been disclosed in Japanese Patent Publication Laid Open No. 182594/90. FIGS. 4, 5 and 6 show the conventional catamaran disclosed in Japanese Patent Publication Laid Open No. 182594/90. FIG. 4 is a schematic side view showing one conventional example of catamaran with an integral connecting portion.
FIG. 5 is a schematic side view showing another conventional example of catamaran with connecting portions separated into fore and aft portions. FIG. 6 is a sectional view along the plane of line 3--3. In the above two examples, two single hulls are arranged in parallel in spaced relation to each other and connected by a deck 7. Each of single hull 6 comprises a semisubmerged portion 8 and a connecting portion 9.
The width of vertical transverse cross section of semisubmerged portion 8 is largest near the water level. Hereafter, the width of vertical transverse cross section of semisubmerged portion 8 is simply called "width." The portion where the width of vertical transverse cross section of semisubmerged portion 8 is largest is called "the maximum width portion 17." The semisubmerged portion 8 is formed in a V shape in which the width decreases gradually in the vertical direction from the maximum width portion 17 to the lower end. On the other hand, the width of semisubmerged portion 8 abruptly decreases upward in the vertical direction from the maximum width portion 17. The width is smallest at the joint with the connecting portion 9. Hereafter, the portion where the width of semisubmerged portion 8 is smallest is called "the minimum width portion 18." The width of connecting portion 9 is equal to the width of the minimum width portion 18. The connecting portion 9 is formed so that its width is constant in the upward direction. The connecting portion 9 is also formed so that its width is smaller than the width of the maximum width portion 17. In addition, the connecting portion 9 is formed so that its width increases toward the deck 7 at the part where the connecting portion 9 is attached to the deck 7.
The bow portion of each single hull 6 composing catamarans 15 and 16 consists essentially of the semisubmerged portion 8. Therefore, the top surface 8a of bow portion of semisubmerged portion 8 is not connected to the connecting portion 9, so that the top surface 8a is formed by horizontal place or inclined surfaces such that an apical angle on vertical transverse cross sectional plane is about 15 degrees. In the two conventional examples shown in FIGS. 4, 5 and 6, the width of semisubmerged portion 8 composing each single hull 6 abruptly decreases upward in the vertical direction from the maximum width portion 17 near the water level, as described above. The abrupt decrease in width of semisubmerged portion 8 reduces the stability of ship when the hulls sink into the water, increasing the synchronizing period of oscillation. As a result, oscillation is reduced when the ship navigates on the head sea of short-period waves occurring frequently. Because of the abrupt decrease in width of semisubmerged portion 8 upward in the vertical direction from the maximum width portion 17 near the water level, the compulsory force of wave is reduced, resulting in a decrease in oscillation.
FIG. 7 is a schematic cross sectional view showing the third conventional example of catamaran. In this example shown in FIG. 7, the shape of bow portion of single hull 10 is similar to that of the catamaran 15 of the first example shown in FIG. 4. The bow portion 10a of each single hull 10 protrudes sharply in the third example shown in FIG. 7. This protruding portion easily plunges into waves, so that the forced moment from waves in the pitching direction is reduced. This may have an effect in reducing pitching. When the wave height increases, however, the compulsory force and forced moment of waves increase; therefore, pitching is possibly not always reduced. In the first and second conventional examples shown in FIGS. 4, 5 and 6, there is a problem of submergence into water of the semisubmerged portion 8 caused when the ship navigates at a high speed on the waves if the width of the minimum width portion 18 which connects the semisubmerged portion 8 to the connecting portion 9 is decreased to below a specified value.
On the other hand, if the width gradually decreases upward in the vertical direction from the maximum width portion 17 near the water level, there is a problem of much smaller effect in reducing the oscillation.
There is also another problem of no effect in reducing oscillation if the minimum width portion 18 of the semisubmerged portion 8 is positioned at a height which waves do not reach.
For the conventional catamaran described above, the bow of submerged portion plunges deeply into waves when the ship navigates at a high speed on the waves because the top surface of semisubmerged portion 8 is formed by horizontal plane or inclined surfaces such that an apical angle on vertical transverse cross sectional plane is about 15 degrees. As a result, the ship floats suddenly after plunging, which hinders navigation. It is unknown what grade of decrease in width of semisubmerged portion above the water level is effective in reducing oscillation. For some shape of submerged portion, a stable effect is not always obtained.